This invention relates to a device for controlling a sound field of space in a chamber such as a concert hall by using an acoustic feedback system and, more particularly, to a device of this type capable of reducing coloration and expanding a margin of howling without causing unnaturalness in hearing and also improving diffusion characteristics of the acoustic feedback system.
As a conventional device for controlling diffusion in an inner space of a chamber by a mechanical system, a rotary diffusion plate is generally employed. According to this device, a diffusion plate provided in a chamber is continuously rotated through an attenuation process of a reverberation attenuation waveform to timewise change the boundary condition in the chamber. Frequency, propagation route and level of the mode (i.e., standing wave) of the chamber are thereby changed timewise to bring about a state which is equivalent to increase in the number of the mode and thereby to improve the sound field control capability.
There have also been various acoustic feedback type devices as shown, for example, in FIG. 2 which have electro-acoustically realized a sound field control including prolongation of reverberation. These devices will be referred to as "sound control devices" hereafter. In the example of FIG. 2, a loudspeaker 12 and a microphone 14 are disposed at some interval from each other in a chamber 10. A sound collected by the microphone 14 is supplied through a head amplifier 16 to an FIR (finite impulse response) filter 18 to produce a reverberation signal. This reverberation signal is fed back to the loudspeaker 12 through an amplifier 20 and diffusion of the sound field in the chamber 10 is thereby realized. In such acoustic feedback system or sound field control device, however, delay time caused by the distance between the loudspeaker 12 and the microphone 14 is constant and this gives rise to the problem that a sharp peak appears in the frequency axis and this causes coloration in hearing and howling at the peak position.
In the prior art sound field control devices, it has been attempted to prevent occurrence of a sharp peak in the frequency axis and thereby reduce coloration and expand a margin of howling by shifting a parameter of the FIR filter 18 continuously and randomly on the time axis.
Shifting of a parameter of the FIR filter 18 on the time axis, however, produces a change on the frequency axis and this generates distortion due to frequency modulation which causes unnaturalness in hearing. Moreover, the problem of generation of coloration and howling due to the peak dip in the transmission characteristic between the loudspeaker and the microphone remains unsettled.
It is, therefore, an object of the invention to provide a sound field control device capable of reducing coloration and expanding the margin of howling without causing unnaturalness in hearing and also improving controllability of the device.